The invention relates to a method for producing high-molecular weight polyesters, such as e.g. polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polynaphthalene terephthalate (PEN), polytrimethylene terephthalate (PTT) and/or polyesters of other dicarboxylic acids and diols including copolymers thereof. The invention likewise relates to a device for implementation of this method. This concerns a single-stage tower reactor with which a method, comprising only two reactors, is made available for the total process.
Methods for continuous production of polyesters are known from the state of the art, in which multiple-stage reactor systems are used which comprise three to five different reaction vessels which are connected to each other. In the case of these methods, the polyester formation is implemented in a plurality of reaction steps which are generally configured as stirred vessels, which steps proceed spatially separately from each other: esterification, transesterification, precondensation, polycondensation and polyesterification. The reaction conditions for the esterification are thereby at temperatures between 200 and 280° C. and pressures between 0 and 4 bar, whilst the conditions for the transesterification are normally at atmospheric pressures and at temperatures between 150 and 240° C., dependent upon the initial substances, especially upon the diols. Low temperatures and low pressures are desirable for the methods in order to avoid undesired secondary reactions.
A method for continuous production of high-molecular weight polybutylene terephthalate is known therefore from DE 35 44 551 A1, in which the method is implemented at atmospheric pressure.
It is important with all the methods known from the state of the art to fulfil the following conditions:    I) Suitable process control and configuration of the devices in order to avoid undesired secondary reactions.    II) As rapid as possible removal of the byproducts, such as e.g. water, methanol, THF and acetaldehyde, as a result of which the reaction equilibrium is moved to the right and the main reaction consequently proceeds with precedence.    III) Sparing treatment, i.e. short dwell time with a minimum reaction temperature.    IV) Optimally coordinated pressure reduction and/or respectively temperature increase in order to achieve the greatest possible progress of the esterification/transesterification and also of the polycondensation/polyesterification.    V) High surface formation for improving the reaction kinetics.
These objects can be fulfilled only unsatisfactorily by means of a plurality of successively connected stirred vessels which are normally used according to the state of the art for these processes. Consequently, e.g. the quality of the product or the quantity of undesired byproducts and also the yield can be affected negatively, as a result of which, in addition to further disadvantages, such as energy consumption, maintenance requirements and investment costs, the economic efficiency of these methods is greatly impaired.
Starting from these disadvantages of the state of the art, it was the object of the present invention to provide a method for producing high-molecular weight polyesters using a single, integrated reactor which fulfils the requirements (I) to (V).
This object is achieved by the generic method with the characterising features of claim 1 and also by the generic device with the characterising features of claim 18. The further dependent claims reveal advantageous developments.
According to the invention, a method for continuous production of high-molecular weight polyester is provided, based on the esterification of dicarboxylic acids and/or transesterification of dicarboxylic acid esters with diols in the presence of catalysts with simultaneous formation of a prepolymer and polycondensation thereof into high-molecular weight polyester. The following steps thereby characterise the course of the method:    a1) A paste and/or a suspension of the dicarboxylic acids and of the diol is produced, a molar ratio of diol to dicarboxylic acid of 0.8 to 1.8 being maintained. At the same time, the temperature is maintained between 20 and 90° C. and the pressure between 0.1 and 1 bar.    a2) Alternatively to a1), a dicarboxylic acid ester can be melted and mixed with the diol in a molar ratio of diol to dicarboxylic acid ester of 1.2 to 1.8 at a temperature of 145 to 165° C.    b) The products from a1) and/or a2) are supplied continuously to a tower reactor. In the tower reactor, the esterification or respectively transesterification is then effected continuously, simultaneously and without interruption until precondensation, the following reaction conditions being maintained:    b1) The products from a1) and/or a2) are subjected to a temperature treatment at 170 to 270° C. and pressures of 0.3 to 3 bar without mechanical stirring. At the same time, a partial quantity of the catalyst and/or catalysts is supplied, whilst the reaction vapours are removed in situ from the reaction mixture.    b2) The product obtained from b1) is transferred continuously into a pipe stretch and the pressure is thereby maintained between 1 and 10 bar, the temperature between 200 and 280° C. In this reaction step, at the same time 0.03 to 0.3 mol diol are supplied.    b3) The product from b2) is transferred continuously into a third zone, a reduced pressure of 0.1 to 2 bar and a temperature between 230 and 280° C. being maintained. 0.02 to 0.2 mol diol and a partial quantity of the catalyst and/or catalysts are supplied in this third step.    b4) The product from b3) is subjected to further reaction, the pressure being maintained between 0.01 and 0.1 bar and the temperature between 240 to 280° C.    b5) The reaction product water from the esterification or respectively methanol from the transesterification, the byproducts and also excess diol from the reaction steps b1) and b3) to b5) are removed and the diol is supplied back to the individual process steps after removal of low boilers.    c1) The prepolymer obtained from b4) is further processed in a normal polycondensation apparatus at temperatures between 240 and 290° C. and at pressures between 0.0002 to 0.003 bar continuously to obtain a polymer.    c2) Alternatively to c1) it is also possible to solidify the obtained prepolymer, to process it into pellets and to subject it to subsequent polycondensation in solid phase at temperatures between 160 and 230° C. under inert gas.